The present invention relates to systems, apparatuses and methods for gem color measurement and analysis, and more particularly systems, apparatuses and methods for measuring and analyzing the color of a diamond in a manner that approximates visual measurement and analysis methodology.
Diamonds and other gemstones are often analyzed based upon their visual appearance to the human eye. Indeed, a diamond""s visual appearance to the human eye under natural or daylight-approximating light is a primary indicator of the quality of the diamond. Accordingly, because diamond quality is substantially based on human visual perception, diamond analysis requires the exercise of judgment, the formation of opinions and the ability to draw fine distinctions based on visual comparisons.
In practice, diamond quality analysis is optimally performed by a team of trained individuals who visually inspect a diamond for features such as inclusions and structural flaws. This time-intensive process involves numerous inspections, measurements and checks by each individual. The process also involves quality control and may include a variety of non-destructive tests to identify treatments, fillings or other defects that may affect the quality of a specimen. Finally, the process includes intensive visual comparison of the diamond with a reference set of diamond master stones that serve as a historical standard with respect to diamond color and clarity.
The foundation of diamond analysis comprises analysis of the Four C""s (color, clarity, cut and carat weight), a method of analysis invented by the Gemological Institute of America (GIA). Two of the Four C""s, color and clarity, are evaluated along a scale or continuum. In the case of colorless to light-yellow colored diamonds, an analysis is made along what is commonly referred to as the GIA D to Z scale. The GIA D to Z color scale, ranging from colorless to yellow, is an international standard which has been calibrated to GIA""s master diamonds since its development.
As described above, the visual inspection process of diamond analysis is subtle, time consuming and requires trained and experienced individuals. As a result, many members of the jewelry and gem field have stated a need for an instrument that can approximately analyze a diamond""s color according to the D to Z standard. Over the years, numerous mechanical instruments have been proposed to xe2x80x9cmeasurexe2x80x9d the color of a polished diamond. Yet, apart from the problems of calibration errors and electronic drift, these instruments have not reached the desired level of accuracy and repeatability due to various inadequacies. Moreover, these instruments have not approximated visual color analysis methodology in a manner that makes their results meaningful within the context of the historical analysis standards.
The history of mechanical gemstone color grading instruments dates at least back to the 1940""s when Dr. Robert M. Shipley, founder of GIA, developed a simple calorimeter comprising a light source and an incrementally moveable, colored plastic wedge. The Shipley device positioned the colored wedge behind a static diamond mount, allowing the user to compare the color of the diamond against the colored wedge backdrop. The Shipley device thus acted as an aid to visual inspection, relying on the human eye instead of a mechanical light detector and the human brain instead of an optical measurement device and processor.
In the 1950""s, Dr. Shipley invented the first non-visual gem color analysis instrument, a modified color comparator, comprising a tungsten filament lamp for a light source, a photo cell for a light detector, blue and yellow light filters, a static stone holder and an iris diaphragm that passes light through to the photo cell. According to the method of the Shipley Colorimeter, the instrument user placed the diamond table down over a diffuser plate so that the tungsten light was transmitted first through the diamond and then through the iris diaphragm to the photocell. The instrument user then made sequential measurements of transmitted light, first deploying the blue filter and then deploying the yellow filter. According to the method of the invention, the instrument user subsequently compared the two transmission magnitudes detected by the photocell and looked up the results in a table organized along the D to Z scale to determine the index of the diamond""s color.
Although the Shipley Colorimeter provided a type of color standard for many years, that standard did not precisely correlate with historical visual analysis standards for several reasons. First, the geometric relationship between the diamond, the light source and the detector did not approximate that of visual diamond analysis. Second, the tungsten filament lamp, though of fair output stability, did not provide the type of daylight conditions which have been the standard for visual analysis of diamonds and other gemstones. Third, the photocell detector did not register each individual frequency in the spectrum of visible light, like the human eye, but rather tracked the change in an overall spectrum magnitude resulting from the change in light filters. Thus, although the Shipley Colorimeter provided a highly useful and innovative instrument for non-visual diamond color analysis, it did not precisely approximate visual analysis methodology. Moreover, during analysis, the diamond was static, not rotated, and the device did not average color over a 360xc2x0 rotation.
In the 1970""s, the Eickhorst Colorimeter and the Okuda Colorimeter introduced two new varieties of color measurement instruments. Although still based on the color comparator method of the Shipley Colorimeter, Eickhorst disclosed the concept of using a fiber optic couple to direct light to the light detector. Okuda disclosed the concept of a voltage-stabilized tungsten light source and an integrating sphere to direct light on the diamond. However, like the Shipley Colorimeter, these instruments relied on the tungsten filament lamps for their light source. Moreover, like the Shipley Colorimeter, the devices compared the overall magnitude of light transmitted by the diamond in response to the use of two different frequency filters. The Eickhorst and Okuda instruments furthermore directed the tungsten light into the crown facets of the diamond, rather than illuminating the diamond into its pavilion side, and did not subsequently measure the light coming out of the pavilion side, as is the case with visual diamond analysis.
In the 1980""s, U.S. Pat. No. 4,508,449, to Okazaki, disclosed an apparatus for measuring the color of a brilliant cut diamond by using a spectrophotometer to measure a limited spectrum of light coming from a diamond. The instrument included an arithmetic unit for deriving tristimulus values X, Y and Z from the measured spectrum. Okazaki further disclosed the use of a xenon or halogen white light source and a type of filter (monochromater) to provide a beam of monochromatic light that is sequentially varied in frequency over a spectral band of interest. Okazaki further disclosed a method of recording the magnitude of light emanating from the diamond in response to sequentially changing frequencies within the spectral band of interest. Okazaki taught away from directing light into the pavilion side of the diamond in the manner of visual analysis (Col. 1, 11.38-39) and did not detect, either directly or indirectly, a specific angle of light coming from the diamond. Additionally, Okazaki""s use of a photomultiplier tube and his sequential measurement of frequency response within the spectral band of interest creates an undesirable time delay in recording the transmission spectrum.
The 1990""s have seen several variations in diamond color analysis instruments. For example, the Austron Colorimeter and the Gran Colorimeter disclosed the use of a photodiode as a light detector. Like their predecessors, the Austron and Gran Colorimeters used, respectively, halogen and tungsten lamps, and directed their illumination into the crown of the diamond. These instruments also did not rotate the diamond during measurement. Additionally, the instruments relied on the colorimeter comparison method of sequential filtering and, in the case of the Gran Colorimeter, compiled tristimulus color values.
Other instruments have incorporated spectrophotometers to improve consistency and accuracy in gemstone color analysis. For example, in 1992 Zeiss-Gubelin developed an instrument using a spectrophotometer. The Zeiss-Gubelin system transmitted light from a Xenon flash lamp via an integrating sphere into the pavilion facets of the diamond, placed table down, and indirectly detected a compilation of light coming from all angles from the pavilion facets of the diamond using the same integrating sphere. The Zeiss-Gubelin system also used a Xenon flash lamp to make a static measurement of diamond color.
Later spectrophotometric systems, including the Rennilson-Hale Gemstone Colorimeter, the Lamdaspec Spectrophotometer, and the Gran Spectrophotometer (DC2000FS) made use of Tungsten, Halogen and/or Xenon lamps. Although these systems were capable of detecting and analyzing the full spectrum of light, the instruments themselves failed to respect the geometric relationships used for visual diamond analysis. Moreover, none of the systems deployed a dynamic color analysis technique involving a rotation of the diamond. The systems furthermore did not average color over a rotation of the diamond.
An Adamas system, which performs color analysis and deploys a spectrophotometer, has been developed. However, the Adamas system illuminates through the table of the diamond, uses an integrating sphere and analyzes color using a single, static measurement. The instrument further does not approximate visual color analysis methodology, and does not meaningfully correlate the results to historical precedents of visual diamond analysis.
What is needed is a simple system and instrument which reliably and consistently approximates historical visual analysis methodology, including, for example, detection methodology, light source composition, illumination angles, and the use of historical visual analysis standards in order to correlate instrument results with historical precedents. There is further a need for a system and instrument with a stabilized output of daylight-approximating light, which can compensate for electronic drift, and which can reduce the hindering effects of light dispersion and direct reflection which occur when mechanically simulating visual detection methodology. Existing devices are inadequate for these purposes.
The present invention comprises systems, instruments and methods for analyzing gem color in a way which reliably and consistently simulates visual color analysis methodology. The preferred embodiment of the present invention comprises several aspects, including the use of daylight-approximating lamps, such as daylight-approximating fluorescent tubes, and a geometry which simulates the results of visual analysis. In the case of non-fancy-colored diamond analysis, in one embodiment, the system includes three major elements: a daylight-approximating light source that illuminates the pavilion side of the diamond, a light detector which detects light coming out of the pavilion side of the diamond at a specific angle, and an optical measurement device which measures the light detected by the light detector. In another embodiment, a measurement chamber encloses the diamond to be analyzed and the daylight-approximating light source illuminates the pavilion side of the diamond through a diffuser. Although a variety of light detectors and optical measurement devices may be used in the present invention, in a preferred embodiment the light detector comprises a fiber optic cable connected to a diode-array, and the optical measurement device comprises a spectrophotometer. In another preferred embodiment of the invention, the system further comprises a fourth element: an optical analysis mechanism, such as a data processor, that compares measurement data from the optical measurement device to a historical precedent and/or converts the measurement data into CIE color space.
According to one embodiment of the invention, the system includes four elements: a daylight-approximating light source that illuminates the pavilion side of a diamond, a rotor which rotates the diamond during illumination, a light detector and an optical measurement device which measures the light detected by the light detector. In a preferred variation of this embodiment, the optical measurement device measures light which the detector has detected coming from the diamond at a specific angle relative to the table of the diamond during the course of a single rotation of the diamond.
The various elements of the invention disclosed herein may be provided as separate pieces or as a single unit. For example, in the previously described embodiment, the light source may form part of an integrated unit with the rotor or alternately be provided separately. In another embodiment, the light detector and the optical measurement device may comprise an integrated unit with the light source and the rotor. The integrated unit may further comprise an optical analysis mechanism. Likewise, elements of the light detector may be part of the same unit as the optical measurement device, as in the case of a diode-array spectrophotometer, or may be provided separately.
In order to overcome difficulties associated with visual analysis geometry, a preferred embodiment of the invention includes at least one of several innovations to increase system stability and reliability. In a preferred embodiment, the system includes high frequency ballast in order to stabilize the daylight-approximating light source. Likewise, in another preferred embodiment, the system includes a light diffuser between the light source and the gemstone which serves to reduce the hindering effects of dispersion and direct reflections caused by the kind of illumination used in visual color analysis. In another preferred embodiment, the invention includes a mechanism to process a plurality of separate light spectrum measurements taken over the course of rotation of a gemstone. According to this preferred embodiment, the invention comprises a rotor platform that optionally has a stabilization ring to ensure its consistent rotation. In another embodiment, designed to overcome difficulties associated with electronic drift, the invention includes a correct-for-drift feature which may operate either statically or dynamically.
The methods of the present invention generally relate to the use of a system and instrument to analyze the color of a gemstone according to historical precedent. In accordance with one aspect of the method, the method of the invention comprises the steps of illuminating a gemstone with daylight-approximating light, detecting the light coming from a specific angle from the gemstone, measuring the detected light with an optical measurement device, analyzing the measurement data with an optical analysis mechanism, and indicating the color of the gemstone according to historical precedent.
According to another aspect of the method of the invention specifically applicable to diamond analysis, the method comprises the steps of illuminating the pavilion side of a diamond with daylight-approximating light, detecting the light that is coming out of the pavilion side of a table-down diamond at a specific angle relative to the table of the diamond, measuring the detected light with an optical measurement device and comparing the measurement data against historical precedent. According to another aspect of the method of the invention, the method comprises the steps of placing a diamond on a rotor platform, illuminating the pavilion side of the diamond with daylight-approximating light, rotating the rotor platform, detecting the light that is coming out of the pavilion side of the diamond at a specific angle relative to the table of the diamond during rotation, measuring the detected light with an optical measurement device and analyzing the measurement data.
As for systems and instruments, it is one object of the invention to increase the ease by which individuals may obtain substantially reliable gemstone color analysis. It is another object of the present invention to overcome the difficulties associated with the application of visual analysis geometry to a mechanical analysis system. It is another object of the invention to provide a stable and reliable system for gem color analysis. It is another object of the invention to reduce dispersion and direct reflection during gem color analysis.
As for the methods of the invention, it is an object of the invention to provide a method of color analysis performable by individuals with little technical training or experience in gemology. It is a further object of the invention to provide methods which permit such individuals with little technical training or experience in gemology to reliably and consistently obtain semi-automated color analyses according to visual color analysis methodology.